To: hang-gliding
Subject: how is flight possible
From: Davis Straub <72147.3716@compuserve.com>
Date: 26 Nov 93 04:42:41 EST
I was pleased to see the recent posting regarding the nature of flight.
There is a fundamental question that we all have as pilots. How is it
that we are able to fly? Unfortunately, this question has not been
answered well, at least IMHO, in the hundred or so aerodynamic texts that
I have reviewed, read, or skimmed.
Over the last two months I was able to bring two aerodynamists from the
Boeing Company to make presentations to the Cloud Base Country Club here
in Seattle Washington. They were charged with answering the questions -
how do you explain flight to a lay audience of pilots.
Martin Withington - speaking also on chipmunk powered flight - stated that
the basic principal is that the wing forces the air down.
Doug McLean - former world record holder in the penny class rubber band
powered flight and touted by Martin as one of the deep thinkers at Boeing
- stated that the basic principal is that the wing forces the air down.
I hope that this discussion can continue and that we will be able to form
a coherent and correct explanation of flight that we can pass along to our
comrades at the hang gliding schools. To encourage the quest I include a
short version of my best explanation and invite your critique:
How is flight possible? - the real short story
Davis Straub
What are the fundamental physical reasons why a wing (and therefore a hang
glider or paraglider) flies?
A glider is an efficient device for gathering air molecules and forcing
them down. The wing span is long in the direction opposite to the
direction of travel in order to gather as many air molecules as possible.
The wing is streamlined so that it can hold on to the air molecules and
slowly force them down smoothly with as few swirls and eddies as possible
breaking out near the surface of the wings.
Because the wing can force large numbers of air molecules down, it can
slow the gliders descent through the air. Because it presents a low
profile to the air molecules as it moves forward, the wings forward
progress is not unduly impeded. It therefore has the opportunity to
encounter large quantities of air molecules that it will force down.
How does a wing force the air down?
Air molecules near the upper surface of the wing are pulled down following
along the contour of the wing from the top most point on the wing down to
the trailing edge. They therefore gain a net downward momentum.
Air molecules near the lower surface are pushed downward by the approaching
wing and by the air molecules that have already been disturbed by the wing.
Wings at higher angles of attack generate greater lift because they are
able to move the air molecules a greater vertical distance in a shorter
amount of time.
This can only continue up to a certain point as angle of attack increases,
after which the air molecules are no longer able to be guided gently
downward.
Why cant I fly a 4x8 sheet of plywood as well as I can fly my glider?
Leaving aside issues of control, center of pressure and center of mass, it
is because the plywood sheet is not able to smoothly guide the air
molecules that it encounters near its top surface down to its trailing
edge. Because of its sharp edged nose the air over the top surface of the
wing is turbulent and loses much of its downward momentum striking the
upper surface of the plywood.
To: hang-gliding , Dale Slechta
Subject: how is flight possible
From: Davis Straub <71603.1057@compuserve.com>
Date: 03 Jan 94 00:31:48 EST
I had intended to go on to the next step in explaining how flight is
possible, i.e. starting from the fact that the wing forces the air near it
downward, how does it accomplish this feat, but I had such a good time
dealing with the response to my first claim that I thought I would tarry a
while and provide further support for it as well as attack Bernoulli.
This attack on Bernoulli is not frivolous although I could have great fun
at the expense of the poor dead man's supporters.
First let's start with Dr. Munk, as in, "The Principles of Aerodynamics"
by Max. W. Munk, Ph.D, Dr. Eng. Max is a very unusual aerodynamist in
that he writes clearly and with great vigor. As an important pioneer in
the development of aerodynamic theory Dr. Munk is well known to all
trained aerodynamists.
Dr. Munk writes, "An aircraft flying through the air is also supported by
the air, propelled by means of the air, and (unfortunately) retarded in
its progress by the air." "An airplane would never be pressed upward by
the air unless it first pressed the air down. In airplane flight, air
must be deflected downward continuously. Fresh, resting, peaceful air is
continually waked up from its slumber and set in motion down toward the
ground. The air thus disturbed resists that motion, thereby pressing the
airplane upward." "Let us consider the air, cubic foot by cubic foot, or
pound by pound if you prefer. Find out what velocity component each pound
of air had before it came into action, and determine the same component
after the action. The air force of that pound, its component in the
direction considered, is directly proportional to the product of its
weight and velocity change, and inverse to the time or period during which
the change took place...The airplane throws the air down;...The airplane
is not placing itself on the back of a vortex or hanging itself under a
vacuum at the beginning of the flight, like a rider on back of his horse.
That is the correct idea; try to impress it into your mind. If you
succeed, you have learned half of all aerodynamics and perhaps more than
that. Do not feel badly if I have upset your ideas about the vacuum and
the vortex." "The wing lifts 'because there is a vacuum on its top' sounds
as absurd to me as saying 'an automobile needs no horse because its wheels
turn by themselves'." "The wing is carried by the air, and a model vacuum
is formed on its top surface because that air reacts to be accelerated
downward, and kicks back." "Netiher of these two things, the larger
velocity or the smaller pressure (referring to Bernoulli's equation), are
the cause of lift, or of each other, but all are different symptoms of the
same thing - lift caused by the change of motion of the air."
On second thought I will wait a bit for the real onslaught on Bernoulli.
To: hang-gliding
Subject: Bernoulli
From: Davis Straub <72147.3716@compuserve.com>
Date: 01 Feb 94 23:25:25 EST
Cc: "Raymond H. Kraft" , Martin Withington
Bernoulli don't know lift and drag
It is quite a burden that we have placed on this poor Swiss
mathematician's shoulders - the explanation of the forces of flight.
Actually the equation named for him is attributed to another Swiss
mathematician of the seventeen hundreds, Leonhard Euler (for a more
complete story see "Introduction to Flight " , John D. Anderson pages
154-156). And neither Euler nor Bernoulli applied it to the study of
flight.
Indulge me by letting me start off with a brazen statement: Bernoulli's
equation determines that the lift and drag of a wing is zero. Therefore
Bernoulli's equation and the phenomena that are said to arise as a
consequence of the correctness of Bernoulli's equation cannot account for
the aerodynamic forces (lift and drag).
Don't believe me? I suggest that you check out "Fundamentals of Flight",
Richard S. Shevell, Stanford University, 1985, page 123, or Kuethe and
Chow, "Foundations of Aerodynamics: Bases for Aerodynamic Design", 1986,
page 86. This s fact is given a name, "D'Alembert's paradox."
Now let me step back a bit, and state that Bernoulli's equation is applied
to "perfect" fluids, fluids that don't have any friction, inviscid fluids.
When hydrodynamic theory (potential flow theory) is applied to air foils
assuming that the air is an inviscid fluid, the resulting streamlines show
a stall point on the upper surface of the wing. Applying Bernoulli's
equation to these streamlines results in a calculation of zero lift and
zero drag. As this is clearly not the case, this combination of
hydrodynamic theory, Bernoulli's equation and the requirement of an
inviscid fluid are not adequate to form the basis of a theory of flight.
Bernoulli's equation is just an expression of the relationship between the
pressure and velocity of an inviscid fluid. It is usually introduced in
aerodynamics texts after the streamlines around a cylinder or airfoil are
empirically illustrated to show how lift could be generated given these
streamlines. It is not an explanation or theory as to why those
streamlines ar e where they are.
This is my first indictment against the use of Bernoulli's equation as an
explanation. It is not sufficient to determine why the air flows where it
does. My second indictment is that it is not necessary. If one has a
theory of flight (say Navier-Stokes equations or Euler's equations
combined with the Kutta condition, see below) which determines the
position and velocity of the streamlines around an air foil, then you have
determined everything that you need to know. You don't need the extra
baggage of Bernoulli's equation (although it can be used quite
conveniently to calculate the pressure distributuon on an air foil and
therefore its lift).
My third indictment is simply that the condition necessary to use
Bernoulli's equation, inviscid air, is contradicted by the condition
necessary to determine the streamlines around an airfoil, the Kutta
Condition, which requires viscous flow.
Re the Kutta condition, Kuethe and Chow state: "A body with a sharp
trailing edge in motion through a fluid creates about itself a circulation
of sufficient strength to hold the rear stagnation point at the trailing
edge," and the "circulation is fixed by the imposition of an empirical
observation." (page 86)
Both these statements are quite unsatisfying from a theoretical point of
view. They basically state that we don't know why lift and drag are
generated, but they are, so let's get on with the work of calculating
their strength (i.e. engineering and forget physics).
It is possible to get around the basic contradiction above by stating that
the air for the most part is invsicid and Bernoulli's equation can be
applied with good accuracy in the most important range of angle of attack,
i.e. not near the stall point, as long as we assume streamlines as
determined from potential flow theory and the imposition of circulation
required to get the stagnation point t o the trailing edge.
To summarize - It is my feeling that using Bernoulli's equation to explain
why a wing produces lift and drag is inappropriate. It is much more
important to explain why the air flows where and at what speed that it
does and once this is accomplished one may use either Bernoulli or Newton
to explain why this flow creates lift and drag.